Major slits which are 50mm wide and not less than 300mm deep may have a bottom 100mm layer of small pea gravel topped to surface level by sand. In this case lateral water movement can take place in the gravel layer so that sands which have a lesser hydraulic conductivity, say 600mm/hr, are acceptable. It is important to use specialised equipment which simultaneously inserts and fully consolidates the sand in order to discourage migration of particles into the gravel.
Generally speaking, in practice, the cheaper the sand in all-sand slits, the less suitable it is for good drainage.
Having to make a drainage scheme more intensive because the sand is not so good is rarely cost effective.
Appendix 1 HYDRAULIC CONDUCTIVITY
Hydraulic conductivity is the measured rate at which water can pass through an individual granular material such as soil, sand or gravel. It is expressed as the height of a column of water (precipitation) which can pass through a known depth of each material in a given time.
Laboratory results are measured underneath the material. A column on top would be subjected to the pressure of a hydraulic head which would cause a false reading.
The Hydraulic Conductivity figure is
Appendix 2
A brief outline of the mathematical basis of drainage design
(Types of measurements must be consistent, for example, all in metric and all in hours)
A) Maximum length of pipes to discharge points
M = F V S
M = maximum length in m F = pipe manufacturers flow rate in L/hr V = scheme design rate mm/hr S = distance between pipes in m B) Design basis (applies to C and D) Accepting the established formula
S = h2 4K V . .. V = h2 4K S2 . ..
K = V S2 4h2
usually stated as millimetres per hour.
Example - A very good sand could be 1,500mm/hr and a poor sand could be 300mm/hr.
It is not that a single hour’s rain is expected to fall at such high rates, but that such a degree of permeability is required for that volume of water to move quickly downwards and laterally in the slits.
The notations which are applied must be varied for each component part of a drainage design and varied again for each different type of design:
C) Applying the formula for sub-soil Evacuation
There are few natural soils in which intrinsic hydraulic conductivity is sufficiently high to provide drainage at rates appropriate to sportsturf requirements. But whether the soil is natural or ameliorated or imported pre- mixed “sandy loam”, the movement of
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